As is generally known, the so-called horizontal axis wind turbine has widely been put to commercially practical use. A general horizontal axis wind turbine is composed of a rotor, in which at least two blades are attached to a hub in a radial manner; a nacelle, which supports the rotor through the main shaft which is connected to the hub and extends substantially in the horizontal direction; and a tower, which is installed in substantially the vertical direction and supports the nacelle in the manner capable of performing a yaw rotation.
In addition, it has conventionally been performed to provide the horizontal wind turbine with control means such as yaw drive means capable of freely performing the drive control of the yaw rotation of the nacelle, a yaw brake to put a brake on the yaw rotation, and a main shaft brake to put a brake on the rotation of the rotor.
Moreover, many of the present commercial wind turbines are configured in the up-wind type horizontal axis wind turbines. The up-wind type horizontal axis wind turbine is a horizontal axis wind turbine having the configuration of generating electricity by the rotation of the rotor putted on the windward side of the tower.
The design strength of a wind turbine is generally greatly influenced by the load applied at the time of being in the waiting state during a windstorm. It is necessary to set the wind turbine load at the time of a windstorm on the supposition of the concurrence of a stoppage of power supply. In the following, typical prior arts 1-5 pertaining to the waiting method of a horizontal axis wind turbine is described.
[Prior Art 1]
Prior art 1 is a general up-wind stall control wind turbine, which performs waiting at the time of a windstorm by fixing the main shaft thereof with the brake thereof. It is the basis thereof to fix the yaw thereof at the time of waiting. A type of the prior art 1 achieves the reduction of the load thereof by controlling yaw to set the rotor thereof parallel to a wind direction. In the case where the electric power necessary for the yaw control is broken, or in the case where trouble is caused in any equipment pertaining to the yaw control, even if the yaw control is possible, there is the possibility of receiving very strong winds from all directions. Accordingly, it is necessary to design the wind turbine on the supposition of the strong winds from all directions. In the case of a stall control machine, a large load is generally generated in the case of the very strong wind from the front thereof or the back thereof.
[Prior Art 2]
Prior art 2 is a general up-wind pitch control machine, which performs waiting at the time of a windstorm by freely rotating the rotor thereof and fixing the yaw thereof. Some pitch control machines achieve the reductions of their loads by controlling yaw to direct their rotors to the windward side of the rotors. The yaw control premises that the pitch control machines have power sources necessary for the yaw control and each piece of equipment of the machines functions without any trouble. In the case of the pitch control machine, a large load is generally produced at the time of a side wind and a very strong wind from the oblique front or the oblique back. A model A shown in FIG. 3 corresponds to the waiting form of the prior art 2.
[Prior Art 3]
Prior art 3 is an up-wind pitch control machine, which secures the feathering of all blades before reversing the nacelle azimuthal angle thereof by about 180 [deg] by yaw control, and which performs waiting at the time of a windstorm by holding the blades with a weak yaw brake (see, for example, a non-patent document 1 and a patent document 2). The rotor thereof thereby swings to the leeward at a windstorm, and the load to the tower thereof can be reduced. The waiting form of the prior art 3 is apparently the same as that of prior art 5. A model B shown in FIG. 3 corresponds to the waiting forms of the prior art 3 and 5.
[Prior Art 4]
Prior art 4 is a downwind wind turbine pitch control machine, which secures the feathering of all blades, and freely rotates the rotor thereof to allow the rotor to be in the state of a free yaw. Then, the prior art 4 performs waiting in that state at the time of a windstorm. The rotor thereby swings to the leeward at the time of a windstorm, and the load operating to the peak of the tower thereof can be reduced. A model C shown in FIG. 3 corresponds to the waiting form of the prior art 4.
[Prior Art 5]
The prior art 5 is described in a patent document 1, and is a downwind wind turbine pitch control machine. After the prior art 5 has secured the feathering of all the blades thereof, the prior art 5 changes the pitch angle of each blade by about 180 [deg] one by one, and performs waiting in the state of a free yaw at the time of a windstorm. The maximum lift coefficient considerably decreases in the case where the wind turbine receives a wind from the trailing edges of the blades thereof in comparison with the case of receiving the wind from the leading edges of the blades, and the yaw holding torque thereof is also small. Consequently, the loads to be produced at the other parts thereof also become small. The waiting form of the prior art 5 is apparently the same as that of the prior art 3. The model B shown in FIG. 3 corresponds to the waiting forms of the prior art 3 and 5.    Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2006-16984    Patent Document 2: WO 2003/58062    Non-Patent Document 1: Shibata Masaaki, Hayashi Yoshiyuki, “New Concept for Reducing Design Load,” 25th Memorial Symposium of Wind Energy Utilization, Nov. 20, Hei. 15, pp. 225-227.